Iron-Based Powder Combination

ABSTRACT

The invention relates to a powder metallurgical combination comprising an iron-based powder A essentially consisting of core particles of iron pre-alloyed with molybdenum and having 6-15%, preferably 8-12% by weight of copper diffusion alloyed to the core particles, an iron-based powder B essentially consisting of particles of iron pre-alloyed with molybdenum and having 4.5-8%, preferably 5-7% by weight of nickel diffusion alloyed to the core particles, and an iron-based powder C essentially consisting of particles of iron pre-alloyed with molybdenum. The invention also relates to the powders A and B per se. Further the invention relates to a method for preparing an iron-based sintered component comprising 0.3-2% by weight of molybdenum, 0.2-2%. Preferably 0.4-0.08% by weight of copper and 0.1-4% by weight of nickel and to a method to obtain a sintered component having a predetermined strength and a predetermined dimensional change during sintering.

FIELD OF THE INVENTION

The present invention refers to iron-based powder metallurgical combinations and to methods for preparing sintered powder metallurgical components therefrom. More specifically the invention refers to the production of sintered components including copper, nickel and molybdenum by using these combinations.

BACKGROUND OF THE INVENTION

Within the powder metallurgical field, copper, nickel and molybdenum has since long been used as alloying elements in the production of high strength sintered components.

Sintered iron-based components can be produced by mixing alloying elements with the pure iron powders. However, this may cause problems with dust and segregation which may lead to variations in size and mechanical properties of the sintered component. In order to avoid segregation the alloying elements may be pre-alloyed or diffusion alloyed with the iron powder. In one method molybdenum is pre-alloyed with iron powder and this pre-alloyed iron powder is subsequently diffusion alloyed with copper and nickel for production of sintered components from iron-based powder compositions containing molybdenum, nickel and copper.

It is however obvious that, when producing a sintered iron-based component, from a powder wherein molybdenum is pre-alloyed and wherein copper and nickel are diffusion alloyed, the content of the alloying elements in the sintered iron-based component will be substantially identical with the content of alloying elements in the used diffusion alloyed powder. In order to reach different contents of the alloying elements in the sintered component, yielding different properties, iron-based powders having different contents of the alloying elements have to be used.

The present invention provides a method of eliminating the need of producing a specific powder for each desired chemical composition of the sintered iron-based component having alloying elements from molybdenum, copper and nickel. The invention also offers the advantage of providing a method for controlling the dimensional change and the tensile strength to predetermined values. In a specific embodiment the dimensional change is independent of the carbon content and the density.

SUMMARY OF THE INVENTION

In brief the invention concerns a powder metallurgical combination of three different iron-based powders. The first of these iron-based powders consisting of core particles of iron, pre-alloyed with molybdenum, which is additionally diffusion alloyed with copper and the second iron-based powder consisting of core particles of iron, pre-alloyed with molybdenum, which is diffusion alloyed with nickel. The third iron-based powder essentially consists of particles of iron pre-alloyed with molybdenum.

The invention also concerns the two diffusion alloyed iron-based powders.

A method according to the invention comprises the steps of combining these three iron-based powders in predetermined amounts, mixing the combination with graphite, compacting the obtained mixture and sintering the obtained green body.

Another aspect of the invention concerns a method of providing a sintered component having a predetermined strength and a predetermined dimensional change during sintering.

DESCRIPTION OF THE DRAWINGS

FIG. 1-4 illustrate diagrams for determining the copper and nickel content in the powder metallurgical combination for a predetermined strength and dimensional change.

DETAILED DESCRIPTION OF THE INVENTION

Specifically the iron-based powder metallurgical combination according to the invention comprises:

-   -   an iron-based powder A essentially consisting of core particles         of iron pre-alloyed with molybdenum, whereby 6-15%, preferably         8-12% by weight of copper, is diffusion alloyed to the core         particles.     -   an iron-based powder B essentially consisting of core particles         of iron pre-alloyed with molybdenum, whereby 4.5-8%, preferably         5-7% by weight of nickel, is diffusion alloyed to the core         particles, and     -   an iron-based powder C, essentially consisting of particles of         iron pre-alloyed with molybdenum.

The amount of pre-alloyed molybdenum in the particles in the iron-based powders A, B and C, respectively, may vary between 0.3-2% by weight, preferably 0.5 and 1.5% by weight. In one embodiment the particles in all three powders are pre-alloyed with the same amount of molybdenum. Amounts above 2% of Mo does not give an increase of the strength justifying the increase of the costs. Amounts of Mo below 0.3% does not give a significant effect of the strength.

The amount of copper and nickel which is diffusion alloyed to the core particles is limited in the upper range to 15% copper and 12% nickel. The lower limit of copper and nickel which is diffusion alloyed to the core particles should be substantially higher than the amount required in the sintered component to achieve the advantages of the invention. Thus, for practical reasons an iron-based powder essentially consisting of core particles pre-alloyed with molybdenum and comprising at least 6% copper diffusion alloyed to the core particles and an iron-based powder having core particles pre-alloyed with molybdenum and comprising at least 4.5% nickel diffusion alloyed to the core particles are of special interest.

The powders A, B and C, respectively, essentially consist of particles of iron pre-alloyed with molybdenum, but other elements, except unavoidable impurities, may be pre-alloyed to the particles. Such elements may be nickel, copper, chromium and manganese.

In order to produce a sintered component from the powder combination according to the present invention, the respective amounts of powder A, B and C are determined and mixed with graphite in the amount required for the predetermined strength. The obtained mixture may be mixed with other additives before compaction and sintering. The amount of graphite which is mixed in the powder combination is up to 1%, preferably 0.3-0.7%.

Other additives are selected from the group consisting of lubricants, binders, other alloying elements, hard phase materials, machinability enhancing agents.

In accordance with one embodiment of the powder metallurgical combination, powder C is essentially free from Cu and Ni.

The relation between powder A, B and C is preferably chosen so that the copper content will be 0.2-2% by weight, the nickel content will be 0.1-4% by weight and the molybdenum content will be 0.3-2% by weight, preferably 0.5-1.5% by weight of the sintered component.

In one embodiment the copper content is 0.2-2%, preferably 0.4-0.8% and the nickel content is 0.1-4%. It has unexpectedly been found that in this particular embodiment the dimensional change during sintering is independent of the carbon content and sintered density.

In order to produce a sintered component with a predetermined dimensional change and strength, the amounts of copper, nickel and carbon, respectively, in the sintered component is determined by means of diagrams, e.g. from FIG. 1-4. The required amounts of powder A, B and C, respectively, may then be determined by a person skilled in the art.

The powders are mixed with graphite to obtain the final desired carbon content. The powder combination is compacted at a compaction pressure between 400-1000 MPa and the obtained green body is sintered at 1100-1300° C. for 10-60 minutes in a protective atmosphere. The sintered body may be subjected to further post treatments, such as heat treatment, surface densification, machining etc.

The exemplifying diagrams in FIG. 1-4 are valid at a compaction pressure of 600 MPa, sintered at 1120° C. for 30 minutes in an atmosphere of 90% nitrogen and 10% of hydrogen.

According to the present invention sintered components containing various amounts of molybdenum, copper and nickel may be produced. This is achieved by using a combination of three different powders, which are mixed in different proportions to achieve a powder having the required chemical composition for the actual sintered component.

To summarize a particular advantage of the invention is that the dimensional change during sintering as well as the strength of the sintered component can be controlled. The advantage of being able to control the dimensional change will facilitate the use of existing pressing tools. When producing sintered parts a certain scatter in carbon content and density may be unavoidable. By utilising the combinations having a dimensional change independent of the density and carbon content the scatter in dimensions after sintering will be reduced hence subsequent machining and machining costs can be decreased.

The invention is illustrated by the following non-limiting examples:

EXAMPLE 1

This example demonstrates how to choose an alloying composition having a desired strength of about 600 MPa and three levels of dimensional change (−0.1%, 0.0% and +0.1%). This was done for two carbon levels, 0.5% C and 0.3% C, respectively, in the powder combinations according to table 1, where the lower carbon content yields better ductility as can be seen in table 2.

The powder combinations according to the present invention were prepared from a powder A with 10% of copper diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum, a powder B with 5% of nickel diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum and a powder C of an iron-based powder pre-alloyed with 0.85% of molybdenum.

The powder combinations were mixed with 0.8% amide wax as a lubricant and graphite, to yield a sintered carbon content of 0.3% and 0.5%, respectively. The obtained mixtures were compacted to tensile test specimen according to ISO 2740.

The compaction pressure was 600 MPa and the sintering conditions were: 1120° C., 30 min, 90% N₂/10% H₂. In table 2 other mechanical properties from the powder combinations according to the invention are presented. It can be clearly seen that the powder combinations according to the invention have the predetermined dimensional change according to FIG. 3. TABLE 1 Dimen- Sintered sional Cu Ni Mo C density change (%) (%) (%) (%) (g/cm³) (%) Powder combination (1) 0.6 1.3 0.83 0.5 7.08 −0.104 Powder combination (2) 1.15 0.8 0.83 0.5 7.06 0.004 Powder combination (3) 1.55 0.4 0.83 0.5 7.04 0.096 Powder combination (4) 0.9 2.3 0.83 0.3 7.11 −0.096 Powder combination (5) 1.3 2 0.83 0.3 7.09 0.007 Powder combination (6) 1.6 1.7 0.83 0.3 7.07 0.095

TABLE 2 Elon- Hard- Tensile Yield Young's ga- ness strength strength modulus tion HV10 (MPa) (MPa) (GPa) (%) Powder combination (1) 219 599 413 139 2.0 Powder combination (2) 223 601 429 139 1.8 Powder combination (3) 219 602 447 139 1.6 Powder combination (4) 207 601 397 138 2.4 Powder combination (5) 209 604 408 137 2.2 Powder combination (6) 206 602 417 137 2.1

EXAMPLE 2

This example illustrates powder combinations according to the invention, comprising 0.6% Cu and 2% Ni and a specific embodiment having dimensional change independent of carbon content and sintered density as shown in table 3. The results obtained with these combinations are compared with the results obtained with Distaloy AB (available from Höganäs AB, Sweden) as well as with a powder having the same chemical composition as the powder combination according to the invention but wherein iron-based powder pre-alloyed with molybdenum has both copper and nickel diffusion alloyed to the surface, in table 3 designated as “fixed composition”.

The powder combinations according to the present invention were prepared from a powder A with 10% of copper diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum, a powder B with 5% of nickel diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum and a powder C consisting of an iron-based powder pre-alloyed with 0.85% of molybdenum.

Table 3 shows a specific example where a mixture of powder A, powder B and powder C having a total content of 0.6% copper, 2% of nickel and 0.83% of molybdenum is compared with a known powder, Distaloy AB, and an iron-based powder having 0.83% of pre-alloyed molybdenum, 0.6% of copper and 2% of nickel diffusion alloyed to the surface of the iron-based powder. As disclosed in table 3 the dimensional change of sintered samples, produced from the powder combination according to the invention, is essentially independent of the carbon content and density compared with the known powder Distaloy AB or the iron-based powder diffusion alloyed with both copper and nickel.

The powder combinations were mixed with 0.8% amide wax as a lubricant and graphite, to yield a sintered carbon content according to table 3. The obtained mixture were compacted to tensile test specimen according to ISO 2740 at different compaction pressures according to table 3. The tensile test specimen were sintered at 1120° C. for 30 minutes in an atmosphere of 90% nitrogen and 10% of hydrogen. In table 4 further mechanical properties are presented. TABLE 3 Compacting Sintered Dimensional Cu Ni Mo C pressure density change (%) (%) (%) (%) (MPa) (g/cm³) (%) Powder combination (7)* 0.6 2 0.83 0.38 600 7.11 −0.117 Powder combination (8)* 0.6 2 0.83 0.54 600 7.09 −0.118 Powder combination (9)* 0.6 2 0.83 0.74 600 7.06 −0.117 Powder combination (10)* 0.6 2 0.83 0.55 400 6.77 −0.114 Powder combination (11)* 0.6 2 0.83 0.53 800 7.22 −0.129 Fixed composition (1) 0.6 2 0.83 0.21 600 7.16 −0.155 Fixed composition (2) 0.6 2 0.83 0.50 600 7.12 −0.147 Fixed composition (3) 0.6 2 0.83 0.78 600 7.08 −0.118 Fixed composition (4) 0.6 2 0.83 0.21 400 6.79 −0.134 Fixed composition (5) 0.6 2 0.83 0.49 800 7.26 −0.163 Distaloy AB (2) 1.5 1.75 0.5 0.35 600 7.06 −0.012 Distaloy AB (3) 1.5 1.75 0.5 0.54 600 7.05 −0.034 Distaloy AB (4) 1.5 1.75 0.5 0.73 600 7.04 −0.056 Distaloy AB (5) 1.5 1.75 0.5 0.54 400 6.73 −0.048 Distaloy AB (6) 1.5 1.75 0.5 0.53 800 7.19 −0.027 *Powder combination according to the invention

TABLE 4 Tensile Yield Young's Hardness strength strength modulus HV10 (MPa) (MPa) (GPa) Elongation (%) Powder combination (7)* 183 570 391 137 2.6 Powder combination (8)* 206 632 433 135 1.8 Powder combination (9)* 244 669 485 138 1.1 Powder combination (10)* 171 507 363 114 1.3 Powder combination (11)* 234 672 450 143 2.1 Fixed composition (1) — — — — — Fixed composition (2) 213 649 437 133 2.2 Fixed composition (3) — — — — — Fixed composition (4) — — — — — Fixed composition (5) — — — — — Distaloy AB (2) 160 562 333 133 3.8 Distaloy AB (3) 189 618 392 136 2.2 Distaloy AB (4) 218 626 437 139 1.1 Distaloy AB (5) 160 523 344 115 1.0 Distaloy AB (6) 200 658 411 145 2.8 *Powder combination according to the invention 

1. A powder metallurgical combination comprising: an iron-based powder A, essentially consisting of core particles of iron-prelloyed with molybdenum, whereby 6-15% by weight of powder A is copper being diffusion alloyed to the core particles, an iron-based powder B, essentially consisting of core particles of iron pre-alloyed with molybdenum, whereby 4.5-8% by weight of powder B is nickel being diffusion alloyed to the core particles, and an iron-based powder C, essentially consisting of particles or iron pre-alloyed with molybdenum.
 2. The powder metallurgical combination according to claim 1, wherein the amount of copper in powder A is 8-12% by weight.
 3. The powder metallurgical combination according to claim 1, wherein the amount of nickel in powder B is 5-7% by weight.
 4. The powder metallurgical combination according to claim 1, wherein the amount of molybdenum in each of powder A, B or C is 0.3-2% by weight.
 5. The powder metallurgical combination according to claim 1, wherein the amount of molybdenum is essentially the same in each of powder A, B or C.
 6. The powder metallurgical combination according to claim 1, wherein the amount of copper in the combination is within the range 0.2-2% by weight.
 7. The powder metallurgical combination according to claim 6, wherein the amount of nickel in the combination is within the range 0.1-4% by weight.
 8. The powder metallurgical combination according to claim 1, further comprising up to 1% graphite by weight.
 9. The powder metallurgical combination according to claim 1, comprising other additives selected from the group consisting of lubricants, binders, other alloying elements, hard phase materials, machinability enhancing agents.
 10. The powder metallurgical combination according to claim 1, wherein powder C is essentially free from Cu and Ni.
 11. A diffusion alloyed iron-based powder essentially consisting of core particles of iron pre-alloyed with 03.-2% by weight of molybdenum, whereby 6-15% by weight of said powder is copper diffusion alloyed to the core particles.
 12. A diffusion alloyed iron-based powder essentially consisting of core particles of iron pre-alloyed with 03.-2%, by weight of molybdenum, whereby 4.5-8% by weight of said powder is nickel diffusion alloyed to the core particles.
 13. A method of preparing an iron-based sintered component comprising 0.3-2% by weight of molybdenum, 0.2-2% by weight of copper and 0.1-4% by weight of nickel comprised mixing powders A, B, C as defined in claim 1 and graphite, compacting the mixture to form a compacted body, and sintering the body.
 14. A method to obtain a sintered component, having a predetermined strength and a predetermined dimensional change during sintering, comprising the steps of: determining the required amounts of copper, nickel, molybdenum and carbon in the sintered component needed for obtaining the predetermined strength and dimensional change, determining the respective amounts of powder A, B and C as defined in claim 1, mixing the determined amounts of powders A, B and C with graphite and optional other additives, compacting the mixture to form a powder compact; and sintering the powder compact.
 15. The powder metallurgical combination according to claim 2, wherein the amount of nickel in powder B is 5-7% by weight.
 16. The powder metallurgical combination according to claim 1, wherein the amount of molybdenum in each of powder A, B or C is 0.5-1.5%, by weight.
 17. The powder metallurgical combination according to claim 1, wherein the amount of copper in the combination is within the range 0.4-0.8% by weight.
 18. The powder metallurgical combination according to claim 1, further comprising 0.3-0.7% graphite by weight.
 19. A diffusion alloyed iron-based powder essentially consisting of core particles of iron pre-alloyed with 0.5-1.5% by weight of molybdenum, whereby 8-12% by weight of said powder is copper diffusion alloyed to the core particles.
 20. A diffusion alloyed iron-based powder essentially consisting of core particles of iron pre-alloyed with 0.7-1.0% by weight of molybdenum, whereby 8-12% by weight of said powder is copper diffusion alloyed to the core particles. 